Satellite television receiving systems usually comprise an “outdoor unit” including a dish-like receiving antenna and a “block” converter, and an “indoor unit” including a tuner and a signal processing section (generally referred to as an integrated receiver decoder (IRD)). The block converter converts the entire range (“block”) of relatively high frequency RF signals transmitted by a satellite to a more manageable, lower range of frequencies.
In a conventional satellite television transmission system television information is transmitted in analog form and the RF signals transmitted by the satellite are in the C (e.g., 3.7 to 4.2 GHz) and Ku (e.g., 11.7 to 14.2 GHz) bands. The RF signal received from the satellite by the antenna of the receiving system are converted by the block converter to the L band (e.g., 900 to 2000 MHz). An RF filter section of the tuner of the indoor unit selects the one of the RF signals received from the block converter corresponding to the selected channel, and a mixer/local oscillator section of the tuner converts the selected RF signal to a lower, intermediate frequency (IF) range for filtering and demodulation.
In newer satellite television systems, such as the DirecTV™ system operated by the Hughes Corporation of California, television information is transmitted in digital form. The RF signals are transmitted by the satellite in the Ku band, and are converted by the block converter to the L band. The frequency range of the RF signals transmitted by the satellite is somewhat smaller (e.g., between 12.2 and 12.7 GHz) than that for the analog satellite television system, and the frequency range of RF signals produced by the block converter is accordingly somewhat smaller (e.g., between 950 and 1450 MHz).
As in the analog satellite television receiving systems, the RF signal corresponding to the selected channel has to be reduced in frequency to an IF frequency range for filtering and demodulation. In a digital satellite receiver, in addition to the normal IF filtering for selecting the desired RF signal and rejecting unwanted RF signals, it is desirable that the IF filter perform what is known as “symbol shaping” to reduce decoding errors due to “inter-symbol interference” caused by bandwidth limitations.
The conversion stage of the block converter of the outdoor unit usually includes a local oscillator which is not stabilized against variations of temperature and age. The result is that the frequency of the local oscillator signal of the block converter changes, causing a corresponding change or offset of the frequencies of the carrier signals of the RF signals received by the tuner of the indoor unit. As a consequence, the frequency of the IF signal produced by the tuner also changes or is offset from its nominal value. If the frequency of the IF signal changes too far from its nominal value, the digital signals modulated on the IF signal cannot be properly demodulated and the information they represent cannot be properly reconstructed. To overcome this problem, the offset frequency is monitored and an offset added to nominal frequency command to change the local oscillator of the tuner to center the signal in the IF filter.
In U.S. patent application Ser. No. 09/155,025, entitled “Tuning System For Achieving Quick Acquisition Times For A Digital Satellite Receiver” filed in the US PCT Receiving Office of the US Patent and Trademark Office on April 5 for John Curtis, III and John Bohach, it is recognized that the RF signals received from the LNB (low noise block) and the corresponding IF signal produced by the tuner may be offset in frequency due to reasons other than a frequency drift of the oscillator of the LNB. More specifically, satellite transponder frequency adjustments may be made by the satellite transmission system operator to reduce the possibility of interference between carrier signals. For example, a transponder frequency may be changed by as much as +/−2 MHz. The transponder frequency adjustments cause the RF signals received from the LNB and the corresponding IF signal produced by the tuner to have a frequency offset.
Accordingly, the method and apparatus described in the Curtis et al. application concern provisions for tuning frequency offsets due to the adjustment of individual transponder frequencies by the satellite transmission system operator. These provisions allow the transmission frequencies of the transponders to be adjusted by the satellite transmission system operator without unduly increasing the time for the indoor unit to acquire the digital signal when a new channel is selected. Briefly, the tuning system measures and stores individual transponder originated frequency offsets. Any offset due to LNB frequency drift is added to all of the transponder frequency offsets as a “global” offset. An individual transponder offset is updated if it is not possible to tune a transponder frequency or if the successful acquisition required a frequency offset greater than a predetermined threshold or is a broad frequency search was required to acquire the signal.
To receive signals from multiple satellites, some receiver systems utilize multiple antennas in combination with multiple LNBs. Differing frequency drift in the various LNBs and frequency offset variations amongst satellites slows the signal acquisition time of the IRD. Limitations imposed on the amount of power that can be carried by the coaxial cable between the IRD and the LNB allow very little power to be sent. As such, only one or two LNB are able to be powered at any one time. Consequently, LNBs must be activated and deactivated to limit the power consumption of the LNBs. With each activation and deactivation the LNB oscillators are allowed to settle before the IRD is tuned. Thus, a substantial amount of time passes each time an LNB is activated. Furthermore, power for the LNB circuits is generally carried from the IRD to the LNBs via a coaxial cable. The amount of power that can be carried to the LNB is limited for safety reasons.
Therefore, there is a need in the art for a satellite receiver that rapidly acquires satellite signals that are received from multiple satellites.